Grain Protein Accumulation and the Relationship between Leaf Nitrate Reductase and Protease Activities during Grain Development in Maize (Zea mays L.)

Abstract

Four maize hybrids, two with high and two with low levels of postanthesis nitrate reductase activity were grown under field conditions. The characteristic enzyme patterns had been established in previous work. Nitrate reductase and proteases were measured in three representative leaves (ear leaf, fourth leaf above and fourth leaf below the ear) at intervals throughout the period of grain development. Concurrent with enzyme sampling, other plants were harvested and subdivided into top, middle and lower leaves, husks, stalks, and ear. Dry weights, nitrate, and reduced N were determined on aDl plant parts for each samping. These data established the rate of N accumulation by the grain and depletion from the vegetative material and provide some insight into the relation between newly reduced and remobilized N and accumulation of grain N. Other plants were harvested at maturity for yield and harvest indices for dry weight and N.Nitrate reductase activity was higher in comparable leaves from the high than from the low nitrate reductase genotypes throughout the grain development period. There was no mathematical correlation between nitrate reductase activity and nitrate content of the leaves or stalks, however the high nitrate reductase genotypes maintained a higher amount of nitrate per plant (largely in the stalk) during the later stages of grain development.From the patterns of plant nitrate content it was deduced that the low nitrate reductase genotypes terminated nitrate absorption sooner than the high nitrate reductase types. Proteolytic activities (casein as substrate at pH 5.5 and 7.5) were higher and increased earlier in the low than in the high nitrate reductase genotypes. The "low nitrate reductase-high protease" genotypes had a higher percentage of grain N, and higher harvest index for N than did the "high nitrate reductase-low protease" genotypes. These results permit the tentative conclusions that: (a) redistribution of vegetative N accounted for more of the grain N in the low than in the high nitrate reductase genotypes; and (b) leaf protease activities are more closely related to the accumulation of grain N than leaf nitrate reductase activity.Hay et al. (16) (7) showed that a significant correlation between NRA and grain N for five wheat cultivars could be obtained if the N translocation efficiency (ratio of grain N to total plant N) was included in the calculations. Therefore, the efficiency of reduced N redistribution is an important regulatory factor in grain N accumulation. In wheat, acid protease activities were correlated with the loss of reduced N from the upper leaves during grain development (8), and Rao and Croy (24) reported that seedling leaf protease activity was higher in cultivars with high grain N compared to low grain N cultivars. Similarly, leaf protease activity and grain N were found to be related in rice cultivars (22). Feller et al. (I 1) showed that in maize leaves, proteolytic activities (casein as substrate at pH 5.4 and 7.5) increased as leaf protein decreased during ...

Proteinases in Senescing Maize (Zea maysL.) Leaves*

“…As reported in our previous papers (FELLER et al, 1977;REED et al, 1980), caseolytic activity at pH 7.5 increased during senescence in maize leaves. Considering the above results, the alkaline protease isolated here may be involved in the degradation of leaf protein during senescence in maize leaves.…”

“…In our previous reports (FELLER et al, 1977;REED et al, 1980) we have shown that protease activity increased during senescence of maize leaves. However the protease had not been characterized.…”

Relationship between Nitrate Uptake, Flux, and Reduction and the Accumulation of Reduced Nitrogen in Maize (Zea mays L.)

“…The high level of leaf blade NRA of hybrid C (Table III) was unexpected as this hybrid had been shown to have low levels of NRA when leaves were assayed during the reproductive phase (19).…”

“…In previous studies with field-grown plants (ref. 19 and unpublished data), the ranking of these four hybrids Plant Physiol. Vol.…”

“…The study presented here was an extension of a preceding field project that utilized the same four maize hybrids (19). The field study was concerned with changes in N metabolism (enzymes and N components) in various plant parts during grain development.…”